In a stackable hub which are made by connecting a plurality of hubs and functionally acts as one hub, a self-number setting system for hub is known (for example, see Patent Literature 1). That system has a hub number receiving means receiving the number of a hub in question from one of hubs cascade-coupled to that hub, a hub number adding means adding a predetermined value to the hub number having been received by the hub number receiving means, a self-number determining means determining the number obtained by the hub number adding means as a self number of that hub, a hub number transmitting means transmitting the self number to the other of cascade-coupled hubs, and a hub number initial value setting means making the number of the hub number receiving means be a constant value when there exists no hub connected to a hub number receiving means side.
Further, there is known an information reception confirmation system where a hierarchical star network that includes one host node and a plurality of lower nodes whose control lines are connected to each other is configured (for example, see Patent Literature 2). The host node has a means for transmitting communication information addressed to the plurality of lower nodes simultaneously. The lower node has a means for transmitting reception confirmation information when receiving the communication information.
Patent Literature 1: Japanese Laid-open Patent Publication No. 07-147586
Patent Literature 2: Japanese Laid-open Patent Publication No. 06-164617
When a plurality of communication devices perform communication unconditionally, congestion of communication sometimes occurs among the plurality of communication devices.